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Suffolk District Attorney Daniel F. Conley assigned his top prosecutor yesterday to review the violent arrest of a teenager last week, whose capture by police was recorded on video by a witness and posted on YouTube.

Conley launched the investigation, a highly unusual move made with the consent of the police commissioner, as Mayor Thomas M. Menino said he was troubled by the video, which shows an officer fiercely punching and kneeing the 16-year-old male.

Still, Menino urged the public not to rush to judgment before the investigation is complete.

“I’m really concerned about the actions of the officers on the video,’’ Menino said in a telephone interview. “The clip that you see shows some perception of excess aggressiveness. But it’s perception. We don’t know if it’s real or not.’’

Police have launched their own internal affairs investigation, but Commissioner Edward F. Davis said a third-party review will ensure the fairness of the probe. First Assistant District Attorney Josh Wall will review the results of the internal investigation to decide whether any criminal charges should be brought against the officers.

The video “obviously raises some questions, some concerns,’’ Conley said. “We’re not prejudging anything. I’m just going to take a good, careful look at this. I thought it was important to get involved right away with and review it with a fresh, independent set of eyes.’’

The release of the video — a seven-minute recording of the Oct. 22 arrest — provoked swift criticism from several community leaders, including some city councilors.

It has also underscored the pressure that new technology has placed on police, whose every move on the street can be recorded by anyone with a cellphone and posted online.

“I am shocked and deeply troubled by what I saw on this video,’’ At-large Councilor Ayanna Pressley tweeted yesterday morning.

In front of police headquarters yesterday, about a block away from where the arrest took place on the Roxbury Community College campus, about a dozen ministers and activists decried the police actions.

With them was Eusida Blidgen, who with her cellphone recorded about seven officers holding down the teenager while one male officer delivers the blows.

The teenager, whose name has not been released, had just fled juvenile custody and violently resisted arrest when officers caught up with him at a college entrance, police have said.

The teenager has a pending charge of assault and battery with a dangerous weapon, according to his criminal record.

“It was just the wrong thing for them to do. The camera doesn’t lie,’’ said Blidgen, a first-year student at the college. “I’m still shaken up by this.’’

Other community leaders, however, said it is too early to judge the police, especially since the video does not show what led to the beating.

“I don’t think at this point we as leaders or the community can say it is police brutality or unnecessary excessive force until we actually know if there was other campus video that shows the event from the beginning,’’ the Rev. Shaun Harrison, associate minister of Charles Street AME Church, said in an e-mail.

Specialists who study police practices said the video does not reveal brutality, but proper procedure.

A sergeant is supervising the officers, who seem relatively calm, and the officer delivering the blows appears to be trying to subdue the suspect, not hurt him, said Thomas Nolan, a former Boston police lieutenant who teaches criminal justice at Boston University.

“Oftentimes, when we see these kinds of videos where the police have gone wild, they’re unsupervised and all hell breaks loose,’’ Nolan said. “The cops are in a frenzy and they’re feeding off each other. We’re not seeing that here. . . . I don’t think [the officer] is doing anything wrong. I think he’s using the amount of force necessary to get this individual to comply.’’

Maria Haberfeld, a professor of police science at John Jay College of Criminal Justice, agreed.

“It looks horrible,’’ she said. “But we constantly forget that the police are authorized to use coercive force to subdue suspects and nobody translates how it looks visually vis-à-vis how it looks on paper.’’

In the video, the suspect is on his stomach, surrounded by officers. The video, shaky at points, makes it hard to see the suspect clearly but at one point he tries to kick the officers.

The teenager had one hand cuffed, but freed it and swung it wildly, a potential danger to the officers, police have said.

In the academy, officers are trained to punch and knee suspects who refuse to be handcuffed on pressure points and fattier parts of the body. The goal is to avoid serious injury and ensure that the suspect cannot reach for a weapon.

If that does not work, officers use nonlethal weapons, like pepper spray or batons.

The next step, and worst-case scenario, is the use of lethal force, Haberfeld said.

She said the video shows that once the teenager was finally in handcuffs, the officers all stepped away. The officers’ names have not been released.

Typically, allegations of excessive force are handled internally by police, who decide whether an officer should be disciplined. But the outcry from some in the community prompted Conley’s investigation, said Elaine Driscoll, police spokeswoman.

“The police commissioner has heard the community’s concerns about their desire for an open and transparent investigation,’’ she said. “He would like to satisfy the concerns.’’

Thomas Drechsler, an attorney with the Boston Police Patrolmen’s Association, said his office is representing officers as they are questioned by Internal Affairs. Several were interviewed yesterday, he said.

Drechsler lashed out at public officials who criticized the officers, specifically City Council president Michael Ross, who on Wednesday said he was outraged by the video.

“Before he makes ill-advised comments and conclusions he really ought to spend the day at the police academy, learning how police officers are trained,’’ Drechsler said.

Ross yesterday said he stood by his comments.

“I do believe [police] work in extreme conditions and have to make split-second decisions that are very hard to make as they’re occurring and easy to criticize after,’’ he said. “But criticize we must if something occurs that’s not right. It was my opinion based on what I saw that there was something wrong with the activity and the behavior of one particular officer.’’

Brian R. Ballou and Milton J. Valencia of the Globe staff contributed to this report. Maria Cramer can be reached at mcramer@globe.com.